Known devices of this type, e.g. as described in German printed specification No. 1,911,469, have a rotor centered on a substantially vertical axis of rotation and provided with radially extending webs or blades between which channels are formed for the circulation of water mixed with air when that member is rotated while being immersed in a body of water to be purified. Conventionally, the rotor comprises a stack of three horizontal annular disks on a central sleeve, the water-circulating channels being divided into an upper and a lower set formed between these disks. The channels, closed on four sides, draw water through the sleeve via their inner ends (proximal to the axis of rotation) and expel it through their outer ends (remote from the axis), thus creating separate flows at two different levels. The upper flow entrains significant quantities of air, depending upon the depth of immersion, which mixes with the lower flow as the two flows intermingle beyond the rotor periphery. Thus, the aeration rate can be varied not only by changing the rotor speed but also by raising or lowering the device with reference to the water level.
Experience has shown that channels open only at their ends tend to clog and can be cleaned only with difficulty, thereby diminishing the efficiency of the aerator. Moreover, changing the depth of immersion allows only a rather coarse adjustment of the aeration rate.